- Tidy up some comments.
- Use correct stack frame linkage everywhere so that if something goes
  wrong, we can get a meaningful back trace.
- Use an additional layer of indirection so that, when we're very
  early in bootstrap, we can just call OpenFirmware directly, rather
  than using our trampoline that saves/restores kernel state.
- Carve out a space for ofwinit() to call into C code to do additional
  initialization.  (This is not done yet.)

(thorpej)